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How Your Joints Work

Topic Overview

Joints occur where at least two bones meet, and they allow the bones
to move with respect to one another. For example, the elbow is the
joint between the one upper and two lower bones of the
arm. The job of a joint is to make repeated movement between the bones smooth,
safe, and efficient.

Joints are made up of several parts, each one needed to allow the
joint to function properly.

Cartilage: The ends of
bones that meet at the joint are covered by a smooth substance (cartilage) that serves both as a shock absorber and as
a tough coat to prevent damage to the underlying bone.

Muscles: Bones are linked together by muscles, strong tissues
that provide the force to move the bones.

Ligaments: Bones are also linked together by
ligaments, strong tissues that form the outer
covering, or capsule, of the joint.

Joint fluid: Specialized cells called synoviocytes that line the inner surface
of the joint capsule fill the
joint with a thin cushion of fluid (synovial fluid),
which absorbs shocks and prevents the bones from hitting each
other.

Bursa: The joint is surrounded by
bursae, fluid-filled pockets that provide buffering
where there might otherwise be friction between the skin and joint, between two
bones, and between a tendon or a ligament and a bone.

The importance of shock absorption

If you have healthy joints, when you stomp your foot down on the
floor, you do not get a sensation of searing pain shooting through your knee.
This is quite remarkable, since the force of your thigh muscles ought to slam
the bones of your upper leg crashing down onto the bones of your lower leg.
This doesn't happen because tissues within the joint serve as shock absorbers,
like springs that absorb energy, and thus slow down the transfer of force from
one bone to the next.

The shock-absorbing properties of the joint are due primarily to the
cartilage and the thin cushion of fluid that fills the space between the
bones.

The cartilage covering the ends of all the bones that meet at a joint
is a Teflon-like substance that is both very hard and very smooth. Cartilage is
made up of cells that receive their nourishment from a solution called synovial
fluid that fills the inside of the joint space. Synovial fluid contains
proteins and sugars and is produced by a layer of cells lining the joint.
Synovial fluid is thick like molasses, enabling it to protect the joint from
transmitting the normal forces associated with movement to the underlying
bones.

The mix of cells and surrounding solution acts much like a sponge.
For example, with each step downward, fluid is squeezed out of the cartilage of
the knee. When pressure is released as the leg comes up off the ground, fluid
rushes back into the cartilage, and it springs back into shape.

Holding the joint together

Joints function as a way to move two bones with respect to one
another. In order for this to work, the bones that meet at the joint must be
attached to each other. The attachment needs to be firm enough to hold the
joint together, yet flexible enough to allow the bones to move. Damage to these
supporting structures of the joint can allow bones to ram into one another,
damaging the smooth cartilage that lines the joint.

Bones are attached by strong bands called
ligaments. Muscles are attached to the bones by bands
called
tendons. Ligaments and tendons are made of tissue that
is strong enough to hold the joint in place but flexible enough not to tear
under normal movement. The placement of tendons and ligaments determines how
different joints are able to move. For example, the knee can bend forward but
not backward. Abnormal movements can lead to damage to these supporting
structures, with long-term consequences for the joint. For example, a common
football injury occurs when a player who has his leg extended forward while
running is contacted on the outer side of the knee by another player running
full tilt at him from the side. The knee, which is supposed to move forward to
back, is suddenly wrenched side to side, tearing the supports.

To keep the skin that covers the joint from restricting movement, the
movements of the bones within the joint must be isolated from the skin. The
structures that separate the joint from the overlying tissues are small, mobile
sacs of fluid called
bursae. Like other components of the joint, damage to
these structures can cause joint pain (bursitis).

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